1. Field of the Invention
The present invention relates to an optical system of a focus detection apparatus, and more specifically to a phase-difference-detection focus detection apparatus which is incorporated in, e.g., a single lens reflex camera or the like.
2. Description of the Prior Art
A focus detection apparatus for use in an SLR (single lens reflex) camera or the like in which a pair of light bundles passed through different portions of an exit pupil of a photographing lens are formed as a pair of images on a corresponding pair of areas on a line sensor (area sensor/photoelectric transducer) via a pair of separator lenses (image reforming lenses) of a focus detection optical system, respectively, to detect the focus state of the photographing lens based on output signals of the line sensor is known in the art (see Japanese examined patent application publication H08-12321). In this type of focus detection apparatus, it is determined that an in-focus state has been obtained when the distance (phase difference) between the pair of images (first and second images) on the line sensor is a predetermined distance with respect to a target object located at a certain distance, or it is determined that an out-of-focus state (front focus state) in which focus is achieved on an object in front of a target object and an out-of-focus state (rear focus state) in which focus is achieved on an object behind a target object have been obtained when the this distance (phase difference) is shorter and longer than the aforementioned predetermined distance, respectively, and thereupon, an amount of focus deviation from the in-focus position is output as an amount of defocus. The space between the pair of images varies according to a variation of the space between the pair of separator lenses of the focus detection optical system. It is common for a pair or pairs of separator lenses of the focus detection apparatus to be integrally molded out of resin for the purpose of miniaturizing the focus detection apparatus, and reducing the production cost thereof.
In the case where resin-made lenses are used as the separator lenses, a temperature variation changes the space between the separator lenses, thus changing the space between the first image and the second image, that are respectively formed on the pair of areas on the line sensor, and also the amount of defocus. Therefore, a configuration which detects the temperature of the focus detection apparatus and corrects the amount of defocus according to the detected temperature of the focus detection apparatus has been proposed in, e.g., Japanese unexamined patent publication S60-235110.
On the other hand, an example which mechanically offsets the variation of the space between the separator lenses, caused by a temperature variation, by a temperature variation of another member has been proposed. For instance, a configuration in which the amount of defocus is maintained constant regardless of temperature variations by varying the distance between the line sensor and the pair of resin-made separator lenses, by warping (caused by a temperature variation) a curved portion of a support member which supports the pair of resin-made separator lenses, has been proposed in Japanese unexamined patent publication H10-68863.
However, in conventional focus detection apparatuses, it is difficult to secure an installation space for a temperature sensor and wiring therefor in the vicinity of a pair of separator lenses (image-reforming lenses) because the separator lenses are usually integrally molded from resin to miniaturize the focus detection system, as mentioned above. There are cases where sensor data on the temperature sensor is used for purposes other than the purpose of detecting focus. In such cases, the temperature sensor and the pair of separator lenses are installed separately from each other with a certain distance thereapart. Even if the sensor data on the temperature sensor is attempted to be used to correct the amount of defocus, detected focus detection data cannot be corrected properly with the sensor data on the temperature sensor because the temperature of the sensing portion of the temperature sensor and the temperature of the separator lenses sometimes do not coincide with each other.
In a method of changing the position of the separator lenses by utilizing variations in shape of a resin-made separator lens support member which are caused by temperature variations, the shape of the separator lens support member varies excessively to thereby make it difficult to maintain optical linearity if the sensitivity to the amount of defocus by variations of the distance between the separator lenses and the line sensor is low. If this sensitivity is high, an extremely high installation accuracy is required, so that the influence of external vibrations or shake easily causes positional shift, etc., of optical members such as line sensors and the separator lenses.